Calendar Calculator

The Calendar Calculator is 100 % web-based, and is therefore compatible with all versions of Windows, Mac, and Linux. It is compatible with most mobile platforms, although the results may not fit on screen withouut scrolling.

Prior to 1582, every year divisible by 4 was a leap year.
Since a year contains only 365.242199 days (slightly less than 365.25 days),
an error of ten days accumulated over the centuries.
To compensate for this error, Pope Gregory XIII
(after whom the Gregorian Calendar is named)
decreed that the ten days between October 5, 1582 and October 14, 1582
would be eliminated from the calendar.
Italy, Spain, and Portugal adopted the Gregorian calendar in October, 1582 as decreed; however, other countries did not adopt this calendar until later.
This made October 1582 the shortest month, with only 21 days.
After 1582, years divisible by 100 are not leap years
unless they are also divisible by 400.
Thus, 1900 is not a leap year, but 2000 is.

The Gregorian Calendar is a vast improvement over the Julian Calendar, which was used previously, but still not perfect.
Note that the Gregorian Calendar made a correction of 10 days in 1582.
However, there were 12 leap years (100, 200, 300, 500, 600, 700, 900, 1000, 1100, 1300, 1400, and 1500) between 1 and 1582
that would not have been leap years had the Gregorian Calendar been in effect.
This is a difference of 2 days (between the 12 leap years and the 10 day correction) over a period of 1582 years.

In fact, correcting for the extra 0.242199 days each year requires 96.8796 days (400 x 0.242199) every 400 years.
The Gregorian Calendar provides for a correction of 97 days (97 leap years) every 400 years.
This is an overcorrection of 0.1204 days every 400 years, or approximately 1 extra leap year every 3322 years.

A modern refinement is that years divisible by 4000 are not leap years. This reduces the error to approximately 1 day every 20,000 years, or one half day through the maximum date (9999) supported by this program.

Although the Gregorian Calendar was adopted in 1582 in Italy, the new calendar was not adopted in England until September 1752.
By this time, another day of error (leap year, 1700) had accumulated, causing the calendars to be 11 days out of synch.
Correcting this error required eliminating 11 days from September in 1752, resulting in a month that was only 19 days long.

As any serious student of history is aware, authoritative information about historical events is sometimes difficult to obtain.
In particular, the dates of adoption of the Gregorian Calendar in different locales are based upon the information that the author was able to obtain from the available References.

The fact that the authority in a particular locale decreed that the Gregorian calendar was to be adopted on a particular date is no guarantee that was aware of or complied with this decree.
Therefore, historical dates are only as trustworthy as the person responsible for reporting them.
It is possible that Gregorian dates were used in some locales prior to their official adoption; or that Julian dates were used in some locales even after the official switch to the Gregorian calendar.
Some other sources of ambiguity:

The Gregorian Calendar also made the year begin on January 1 rather than March 25 (Around the Vernal Equinox). Therefore, there is some ambiguity regarding the year for dates between January 1 and March 25.
(Just as today, the Fiscal Year and the Academic Year do not necessarily coincide with the Civil Year...

For people born shortly after midnight, there is some ambiguity regarding the date.

Some astrologers used astronomical time, which runs from noon to noon.

Each user of the Calendar Calculator is ultimately responsible for evaluating the reliability of the results.

The original Perpetual Calendar was written in Java in March, 1996 and is still available at Perpetual Calendar.
Sun has promoted Java as Write once, run anywhere.
The fact that this program behaved differently in different browsers, does not even function in some browsers, and will no longer compile using the most recent compiler has failed to win me over as a Java convert.
I subsequently implemented a Perpetual Calendar written in C to provide a more robust solution.

This new Calendar Calculator added several new functions, including the ability to view an entire year at a time,
the ability to calculate the difference (number of days or weeks or years, months, and days) between two dates,
the ability to calculate a date some number of days or weeks or years, months, and days before or after another date, and the ability to convert a date from one locale to another.

Astronomical Time: mean solar time reckoned by counting the hours continuously up to twenty-four from one noon to the next.

Civil Reckoning: (Popular Reckoning) 1560-61 or 1560/1

Civil Time: time as reckoned for the purposes of common life into days, being divided into two series of hours twelve each, and reckoned, the first series from midnight to noon, the second, from noon to midnight.

Ecclesiastical Reckoning: of or associated with a church (especially a Christian Church). (Legal Reckoning) 1561

Ephemeris: an annual publication containing astronomical tables that give the positions of the celestial bodies throughout the year

Gregorian Calendar: the calendar in common use today, decreed by Pope Gregory XIII in 1582.

Julian Calendar: the solar calendar introduced in Rome in 46 b.c. by Julius Caesar and slightly modified by Augustus, establishing the 12-month year of 365 days with each 4th year having 366 days and the months having 31 or 30 days except for February

Thinking Cap # 34 - Discrepancies in the Bible: 5. Other incongruities arise from the use of different modes of computation, particularly in the reckoning of time. Many nations had two or more kinds of calendars in use including the civil and the sacred. Among the Latin Christian nations there were seven different dates for the start of a year. [Appleton's Cyclopaedia, Calendar]